The present invention relates to burner assemblies and particularly to high capacity tube-fired burners. More particularly, the present invention relates to an immersion tube burner including a combustion chamber for burning a combustible air and fuel mixture and an immersion tube heat exchanger.
Immersion tube burners are used in a variety of industrial processes to heat solution tanks containing liquid. It is often necessary to heat liquids such as water for parts cleaning or chemical baths for parts treating or plating. It is known to mount an immersion tube burner to a liquid-containing solution tank. The burner is arranged so that it fires into one end of a long pipe or serpentine tube which passes through liquid in the solution tank. An outlet end of the tube is connected to an exhaust stack. See, for example, U.S. Pat. No. 4,014,316 to Jones et al.
Typically, tube burners will either use refractory in the combustion chamber or the burner will attach to the wall of the tank so that the combustion chamber is mounted inside the tank. Refractory represents a large initial acquisition expense as well as continuing operating costs due to maintenance and repair. Mounting the combustion chamber in the tank allows the liquid in the tank to provide the cooling necessary to keep the combustion chamber from melting. However, these combustion chambers can range from 8-20 inches (20.3-50.8 cm) in diameter and from 25-52 inches (63.5-132.1 cm) in length. Obviously, such chambers represent a large volume of space consumed in the tank.
Eliminating the combustion chamber from the tank would allow for more passes of a smaller diameter tube through the liquid, thereby increasing the overall thermal efficiency of the apparatus. It also allows the use of a smaller tank with associated floor space savings. Doing away with the refractory would decrease initial acquisition expense, save weight, and eliminate maintenance and repair associated with the refractory.
In the past, in order to fire enough gas to achieve the necessary temperatures, high pressure fans and relatively large diameter tubes were used. See, for example, U.S. Pat. No. 4,014,316 to Jones et al which is designed to use "the highest pressure supply normally available." The high pressure fans, because of the size of the fan and associated ducting, represent another major cost factor in terms of acquisition. The larger fans require larger horsepower motors to drive them, and therefore have higher operating expenses.
The large diameter tubes generally ranged between six inches (15.2 cm) and twelve inches (30.5 cm) in diameter. Large diameter tubes can increase costs by as much as a factor of four over a smaller diameter tube just for straight sections, with curves and bends in the tubes costing even more. However, in the past it has been difficult to maintain flame stability when attempting to burn large amounts of fuel in a small diameter tube.
Recognizing the potential for initial acquisition and operational savings, there is a need for a smaller diameter tube burner operating with a low-pressure combustion air source. Such a burner would allow reduction in size of solution tanks and tubing. It would further allow the use of a smaller fan with a smaller horsepower motor and smaller diameter air ducting. A burner that could meet such demand would represent a substantial improvement over a conventional immersion tube burner.
According to the present invention, a burner assembly for combining air and fuel to produce a burn firing into a downstream tube includes a funnel formed to include an inlet end, an outlet end, and an air and fuel mixing region therebetween. The funnel also has a central longitudinal axis and includes a cylindrical intake end at the inlet end and a conical side wall mating with the cylindrical intake end and converging from the cylindrical intake end toward the outlet end to fire a burn initiated in the mixing region into a tube coupled to the outlet end of the funnel.
The burner assembly also includes means for supplying a gaseous fuel to the mixing region in the funnel and means for introducing combustion air into the mixing region through the inlet end of the funnel. The combustion air mixes with the gaseous fuel in the mixing region to produce a combustible mixture. The introducing means includes an air-mixing plate mounted in the inlet end of the funnel. The air-mixing plate is formed to include a plurality of air supply apertures passing combustion air into the mixing region.
The burner assembly also includes means for igniting the combustible mixture in the funnel to fire a burn into the downstream tube, which tube is coupled to the outlet end of the funnel and extended into the interior solution-containing region of an adjacent solution tank, so that combustion begins, progresses, and transitions gradually into the downstream tube. Thus, the combustion reaction is delayed as only a small stabilizing portion of the fuel begins to burn in the funnel and the rest of the fuel burn is delayed until the air and fuel mixture has exited from the downstream of the funnel and entered into the tube mounted in the solution tank.
In preferred embodiments, the introducing means includes a burner housing formed to include a discharge outlet and an interior region containing combustion air. The funnel is located in the interior region of the burner housing to position the air-mixing plate in the interior region so that combustion air is supplied to the mixing region through the apertures in the air-mixing plate. The outlet end of the funnel is coupled to the discharge outlet of the burner housing so that a burn initiated in the mixing region of the funnel is fired into a downstream tube positioned outside the burner housing and coupled to the outlet end of the funnel through the discharge outlet. The design of the burner makes it well-suited to be located outside of a tank containing liquid to be heated and used to fire a burn into a small bore tube heat exchanger situated in the liquid-containing tank.
Gaseous fuel is discharged into the mixing region in the funnel by a fuel discharge nozzle. The nozzle has an annular side wall and a closed end wall. A portion of the annular side wall of the nozzle is formed to include a plurality of gaseous fuel discharge ports that are arranged to discharge gaseous fuel into the mixing region in the funnel. The air-mixing plate is formed to include a central aperture and the fuel discharge nozzle is mounted in the burner assembly to extend through the central aperture and position the gaseous fuel discharge ports and the closed end wall in the mixing region defined by the funnel.
The air-mixing plate is perforated to include supply apertures for passing combustion air into the air and fuel mixing region defined by the funnel. These apertures are arranged in a pattern designed to permit use of low pressure combustion air and generate a burn that can be fired into a small bore tube heat exchanger. The pattern defines several concentric rings of air supply apertures and calls for the apertures in each ring to be spaced apart uniformly about the circumference of each ring. The apertures in the innermost ring of air supply apertures have the smallest internal diameter and the apertures in the outermost ring of air supply apertures have the largest internal diameter. This unique pattern of air supply apertures allows low pressure combustion air passing through the burner housing and swirling around the funnel to pass through the perforated air-mixing plate into the mixing region provided in the funnel to mix with gaseous fuel discharged into the mixing region by the nozzle so that a stable burn is initiated and supported in the mixing region.
By providing combustion air to a "transition" chamber that is defined by a funnel located inside the burner housing, the present invention channels combustion air to pass over and around the funnel to cool the transition chamber defined by the funnel before it reaches the air-mixing plate. By cooling the transition chamber with combustion air, the present invention allows the transition chamber to be located outside the tank containing liquid to be heated, yet avoids the need to use brittle and expensive refractory surface to define the transition chamber. Removing the transition chamber from inside the liquid-containing tank allows a reduction in size of the tank, tubes, and associated equipment. By allowing the use of smaller diameter heat exchanger tubes in the tank, the present invention also provides increased heat transfer efficiency, thereby providing a substantial improvement over conventional gas-fired tube burners.
By providing an air-mixing plate having apertures of various sizes, the present invention allows a sufficient amount of combustion air to be provided to the air and fuel mixing region in the funnel by a low pressure air fan and eliminates the need for a high pressure air fan of the type that is typically used with a conventional small bore immersion heating system. Use of a low pressure air fan allows the use of a burner with combustion air fan and gas/air control devices integral to the burner unit to eliminate the need for high pressure air ducting. At the same time, the design of the air-mixing plate allows cooling combustion air to pass through the transition chamber along the inner wall of the funnel defining the transition chamber to provide additional cooling of the transition chamber and increase control of the burn. The funnel defines a tapered transition chamber converging from its inlet holding the air-mixing plate to its outlet joining the tube heat exchanger. This funnel converges as a selected angle along its length to allow gradual controlled combustion of the air and fuel mixture to provide a higher burner firing rate into a small bore tube heat exchanger. The funnel provides a firing cone which allows combustion to begin, progress, and transition gradually into a small bore tube heat exchanger having a desired internal diameter.
Another aspect of the invention relates to a fuel supply control valve that is included in the fuel-supplying means to regulate flow of gaseous fuel into the air and fuel mixing region in the burner housing. Instead of using a conventional butterfly valve, a slotted shaft-type fuel supply control valve is used to regulate fuel flow into the burner housing. Such a valve is easy to install and replace. Also, the slot in the valve shaft can be sized and arranged to allow a small flow of fuel to be fed into the air and fuel mixing region when the valve is moved to its generally "closed" position. Advantageously, this feature makes it easy for users of the burner assembly 10 to idle the burner at low fire rates rather than shut off the burner completely and therefore require a later reignition sequence to put the burner back in operation. Illustratively, the cylindrically shaped fuel supply control valve is rotated about its longitudinal axis to regulate the flow of fuel into burner housing 26.